The invention relates to a method and device in a crawling operation of a press felt in a paper machine. In the paper machine, the press felt is passed as a closed loop through a press nip defined between a center roll and a press roll. The drive of the felt is produced by means of a nip drive during a running operation of the paper machine.
When replacing the press felt and installing a new felt, the new felt is advantageously moistened before starting the operation of the paper machine. In this felt-moistening stage, the felt is driven with a low crawling speed by means of a separate drive gear while the press nip is open.
In prior art devices, the drive gear consists of an electric motor whose speed of rotation is lowered by means of a reduction gear. An output shaft of the reduction gear is connected to a mechanical cylinder-operated coupling. The rotational movement of the output shaft is transferred through the coupling to an axle of a drive roll of the felt. The drive gear is connected to the drive roll in order to cause the drive roll to revolve and thus turn the felt so that it can be moistened.
The device described above has been formed by coupling the constructional units of the mechanism one after the other. However, in this manner, the length of the device becomes detrimentally long. Further, the device consists of a number of individual components each one of which has a certain rate of failure. For this reason, the susceptibility of failure of the complete operating mechanism of the device is relatively high.
In the cylinder operation of the coupling, separate limit detectors have been used. By using these detectors in the prior art devices, the main drive of the felt is prevented from switching on in situations where the auxiliary drive, i.e. The nip drive, is still in operation. However, it is a significant drawback that the detectors are susceptible to contamination, in which case the data on limit positions given by the detectors has not always been reliable.